1. Field of the Disclosure
The present application relates to images sensors suitable for detecting images at vacuum UV (VUV), deep UV (DUV), visible and near infra-red (NIR) wavelengths, and to inspection systems incorporating such sensors. In particular it relates to sensors and methods for fabricating sensors with low noise and high sensitivity. The sensors are particularly suitable for use in inspection systems including those used to inspect photomasks, reticles, and semiconductor wafers.
2. Related Art
The integrated circuit industry requires inspection tools with increasingly higher sensitivity to detect ever smaller defects and particles including those with dimensions close to 10 nm or smaller. Furthermore these inspection tools must operate at high speed in order to inspect 100%, or a large fraction, of the area of a photomask, reticle or wafer, usually in less than one hour. Some applications require many (such as about 50 or 100) wafers to be inspected in one hour. Generally short wavelengths such as UV, deep UV (DUV) and vacuum UV (VUV) wavelengths have higher sensitivity for detecting small defects and particles compared with longer wavelengths. Inspection of photomasks or reticles is best done using the same wavelength as used for lithography, which is currently a wavelength of substantially 193.4 nm for the most critical lithography steps and substantially 248 nm for less critical steps. High-speed inspection requires sensors with high sensitivity and low noise in order to detect the small amount of light scattered from small particles or defects or allow detection of small changes in reflectivity due to defects in the pattern. An image sensor that can detect a change in light level of one, or a few, photons is highly desirable.
Silicon CMOS and CCD image sensors are known in the art. CCD image sensors are particularly suitable for high-speed inspection systems for semiconductor wafers, photomasks and reticles because the electronic noise of such sensors is quite low and follows closely a Poisson statistical distribution (apart from very infrequent events caused by absorption of charged particles from cosmic rays or radioactive decay, which are rare and can generally be filtered out by image processing software). Silicon CCD image sensors can have noise levels equivalent to about 2 electrons RMS if the sensor is cooled to about 100° K and is read out at a relatively low speed (such as a rate of a few hundred thousand pixels per second or less) and appropriate driving and reading electronics are used. Such sensors, when operated at similar speeds, but at a temperature closer to room temperature (such as about −10° C.) may have noise levels equivalent to about 5-10 electrons RMS. However high-speed inspection systems require data rates of multiple billions of pixels per second, which are generally achieved by reading many tens or a few hundred pixels (taps) simultaneously at rates of several to a few tens of millions of pixels per second. Such high data rates and so many output channels operating at the same time generate many Watts of heat making cooling below room temperature impractical. The high speed operation itself also generates more electrical noise and, when combined with the high operating temperature, can lead to noise levels equivalent to about 20 to 40 electrons RMS.
CMOS sensors typically have higher noise levels than CCD sensors because CMOS transistors have their channels on the surface of the silicon resulting in noise from the silicon to silicon dioxide interface (due to defects and traps at that interface). Furthermore this noise from the surface defects and traps does not closely follow Poisson statistics. Even if the RMS noise is low, high noise spikes are much more frequent than would be expected from Poisson statistics. This is a serious problem for inspection systems, as these high noise spikes can result in a false detection of a defect. Systems with a CMOS detector may have high rates of reporting false defect rates when operated in their highest sensitivity modes. A reinspection would be needed to separate false from true defects, slowing down the inspection.
For UV wavelengths, when a photon is absorbed in silicon, usually only a single electron-hole pair is created, but occasionally two pairs may be created, resulting in average yield per absorbed photon slightly greater than 1. At DUV and VUV wavelengths, the probability of a second electron-hole pair being produced increases so the average electron yield increases. For example, when photons of a vacuum wavelength of 193 nm are absorbed in silicon, the average yield is about 1.7 electron-hole pairs per absorbed photon. For wavelengths currently used in semiconductor inspection systems and wavelengths likely to be used within the next several years, the electron-hole pair yield will not exceed 2. Hence silicon CCD and CMOS sensors are not able to reliably detect one, or a few, photons when sensing visible, UV, DUV or VUV wavelengths.
Avalanche photodiodes are known in the art. An avalanche photodiode uses a relatively large reverse bias voltage (tens to a few hundred volts) over a distance of about one hundred or a few hundred microns of silicon in order to generate multiple carriers (electrons or holes) from a single carrier created by photon absorption. When a photon is absorbed, an electron-hole pair is created, usually close to the surface when sensing UV radiation because of the strong absorption of silicon at UV wavelengths. The bias voltage accelerates the carriers. When a carrier has accelerated to a high enough speed to have about 3.7 eV of energy, it can create an additional electron-hole pair by collision. This process can be repeated a few times creating more carriers and, hence, a large signal.
Most common avalanche diodes absorb the incident light in n-type silicon and apply a bias voltage to accelerate holes away from the surface. This is because surface defects on the silicon tend to have positive charges and attract electrons. Furthermore, to make an avalanche detector that uses electrons rather than holes requires doping the light absorbing silicon to p-type silicon. Boron is the only practically useful p-type dopant for silicon. Boron diffuses easily into silicon dioxide create positive charges in the oxide. This further increases the electron recombination rate at the surface and makes conventional electron-based avalanche photodiodes less efficient for UV, DUV and VUV wavelengths. The avalanche gain and mobility are both lower for holes than electrons in silicon. So avalanche diodes using holes need a longer length in silicon and/or a higher operating voltage in order to achieve a given gain factor.
Therefore, a need arises for a sensor overcoming some, or all, of the above disadvantages. In particular a need arises for an image sensor that can detect very low levels of UV, DUV and/or VUV light while operating at very high data rates, such as billions of pixels per second.